Send client previously seen observations.

* Redis keyserver now stores a set of UNIX timestamps for each
  event track under observation, the key is stored in the format
  `<EVENT_NAME>_set`.
  Once the `events` set is flushed on restart of the application, a
  given `<EVENT_NAME>_set` will be deleted when that event is first
  re-encountered.
* When a client connects, the application will query for each set of
  timestamps corresponding to all known events and initialise an array
  for each, in which to store a flag for an event occuring in each of
  the last `STEP_SIZE` seconds.
  Note that this list is reversed before sending to the client.
* When a client connects, a client-side flag determines whether to load
  an existing set of observations as sent by the server, or to start
  taking observations from the `event_flags` object.

	modified:   poisson.py
	modified:   templates/poisson.html